The present invention relates to a closed-loop control system, and more particularly, to feedback control systems utilizing adaptive inverse control to adjust output signals in a test system.
A feedback control system operates to achieve prescribed relationships between selected system variables by comparing functions of those variables and using the comparison to affect control. System variables are those quantities or conditions of the system which are subject to change. Examples of such variables include an electrical voltage level generated by an amplifier or physical force applied to a specimen by a servoactuator. Control is the governing of the response of the controlled subsystem such as rotational velocity of an electrical motor, strain in a structural member of a truss, or position of an elevator. A sinusoidal signal amplitude and phase control for an adaptive feedback control system is disclosed in the Thoen U.S. Pat. No. 5,124,626, assigned to the same assignee as the present invention, and herein incorporated by reference.
Changes in operating conditions or in transfer functions of system elements can affect a feedback control system. External disturbances can also affect any system variable. While such external disturbances do not themselves change the transfer function of the system, they can affect system accuracy in following the desired function values. In addition, under the influence of an experiment, physical changes can occur in the controlled subsystem which change the subsystem's transfer characteristics over time. Such changes affect the output response generated by a particular actuation signal and necessitate repeated tuning of the control system in some experiments.
Adaptive inverse control (AIC) is a technique for achieving good tracking response in a closed loop control system. AIC places an adaptive filter between a function generator and a feedback loop comprising a subsystem to be controlled. The feedback loop is commonly referred to as a plant. The purpose of the adaptive filter is to compensate for the plant's frequency response irregularities. If the adaptive filter has a dynamic behavior which substantially matches the inverse frequency response function of the plant, an overall input-output frequency response of unity is achieved over a broad range of frequencies. A key to adaptive inverse control is that a network adjusts the inverse frequency response function applied to the filter while the plant is actually operating, or online. In this way, the controller adapts to changing conditions.